The International Cessna 170 Association

I am having a problem with my 1948 rag-wing. I have replaced the pitot tube, pitot lines, static lines, and had the ASI rebuilt. The ASI calibration was within one mph at all speeds according to Otto Instruments, in Ontario, California.

After three leak checks there was no error in either system, and the airplane meets IFR certification requirements for the pitot/static system and transponder mode C. However, in flight, the ASI consitently reads 10 mph slow at speeds below 100 mph. At speeds above 100 mph the ASI reads 10-15 mph slow. For example: at 60 IAS; the airplane is flying at 70 mph. At 70 IAS; the airplane is flying at 80 mph. At 90 IAS; the airplane is flying at 98 mph. At 100 IAS; the airplane is flying at 115 mph. At 110 IAS; the airplane is flying at 125 mph.

The afformentioned numbers are derived from my Lowrance 2000 GPS which is very accurate. The conditions are no wind and stabalized straight and level flight at 500 AGL and 1000 AGL.

Does anyone have any suggestions?

Could this be caused by pitot tube position error? If so, where can I get the template for pitot tube position and angle on the wing?

Owner of N4236V

At the risk of stating the obvious, this wouldn't be an issue of knots vs. mph would it? At a conversion factor of 1.15, 100 kts = 115 mph. The various numbers you quoted are certainly within a margin of error that could indicate a kts. vs mph situation.

Just a thought...

I'd suspect the airspeed indicator since the system reads low then high. If it was consistently low or high then perhaps the pitot angle would be it.

Just my thoughts.

There is a template for the pitot in the service manual 100 serious '62 and prior. It's for a B model. Pretty sure the airfoil of the rag wing is different. Perhaps the template could give you an idea if you close

Oooh, Doug! That's a good thought. I've seen two airspeed indicators that were incorrectly screened (on aircraft other than 170's.) It can happen apparently.
Another way to get a quick-and-dirty comparison is to fly in formation with another aircraft.

Well, that's my guess, but it is a little confusing since FLight states that the ASI reads fast below 100 mph and low above 100 mph yet all the numbers quoted indicated that the ASI is low in all situations. The ASI being low in all situations is what I based my suggestion on.

He could also test by switching the gps from mph to kts and see if it matches the ASI.

"Indicated airspeed problems?"

Yeah, mine ALWAYS indicates lower than I want...

Thanks guys for the suggestions. Unfortunately, I have done all of the above except fly along side another airplane to compare IAS. I have adjusted the pitot tube angle upwards and downwards. The result of our final "tweaking" of the pitot tube has now brought the spread to 10 MPH lower than what the airplane is actually flying. I am waiting to get a template, because we believe that the tube is to long. I am convinced at this point, our problem is pitot position error.

Flight

flight wrote:Thanks guys for the suggestions. Unfortunately, I have done all of the above except fly along side another airplane to compare IAS. I have adjusted the pitot tube angle upwards and downwards. The result of our final "tweaking" of the pitot tube has now brought the spread to 10 MPH lower than what the airplane is actually flying. I am waiting to get a template, because we believe that the tube is to long. I am convinced at this point, our problem is pitot position error.

Flight

Highly unlikely that your problem is position error or length of the tube. Airspeed works on difference between pressure at pitot and pressure at static port. Look to your static port lines for any blockage or leaks. You say the system has been IFR certified, leak checked and the ASI read correctly with the tester. Think about it. There is no way you are going to generate more pressure than the speed you are flying at. The only way for the system to read high is for the static system to have too low a pressure, assuming ASI is calibrated and correct. Optimal positioning of the pitot gets you the maximum pressure, you can't make it any higher.
I'm not familiar with the static port position on the ragwings. On the A and B model it is riveted to the backside of the fuselage skin, and that joint is notorious for leakage. Changing cabin pressure would impact altitude and airspeed indications if leaking there.

Kelly,

I agree with what you are saying; we first thought the problem was with the static system, or static port-and it still may be. We checked for leakage around the port itself and the plate inside the fuselage. On the rag-wing, the port is located just forward of the left door frame. We are exploring laminar flow problems around that area.

You are also right about dynamic pressure at the pitot tube. Our problem is: when adjusting the pitot angle, we can get the error to zero out at 100 mph or near cruise flight attitude, but as I increase angle of attack or decrease angle of attack from that point the error begins to increase. We realize at very high angles of attack, pitot position error occurs. However, as you stated, that error is insignificant within the normal flight envelope.

You sound astute on this subject. The fellows helping with this matter are the former Dixie College Aviation Department heads. They are both A&Ps and pilots (they are familiar with both maintenance and flight). We still believe there might be a leak in the static system that is elluding us, because the static leak check on the ground is positive; yet in the air when the cabin pressure could effect static pressure if there is a leak, the error appears.

Until we check the pitot tube position with the template, we cannot rule out some position error. I do agree that the pitot tube can only provide a given amount of dynamic pressure in it's optimum position. At this point, we are using the process of elimination.

Thank-you for your insight. I know with persistence, we will find the problem. I will certainly let all of you know what we discover.

Flight

Go to the attached link and see what the Cessna 120-140 Assc has to say about the pitot static problems. It seems there are things that Cessna told it's factory dealer/mechanics and didn't pass along to the owners about pitot-statc problems. This could be your problem. It fixed my buddy's 140 2 weeks ago. I was always indicating 105mph in my 170B and we would be in formation and his read 85-90mph. Pulled the line off the transponder after reading the article. His now works fine. And yes, he know it will not past the 24 month pitot-static test.
http://www.cessna120-140.org/forum/view ... bf8f73ccc4

flight wrote:Kelly,

You are also right about dynamic pressure at the pitot tube. Our problem is: when adjusting the pitot angle, we can get the error to zero out at 100 mph or near cruise flight attitude, but as I increase angle of attack or decrease angle of attack from that point the error begins to increase. We realize at very high angles of attack, pitot position error occurs. However, as you stated, that error is insignificant within the normal flight envelope.

You sound astute on this subject. The fellows helping with this matter are the former Dixie College Aviation Department heads. They are both A&Ps and pilots (they are familiar with both maintenance and flight). We still believe there might be a leak in the static system that is elluding us, because the static leak check on the ground is positive; yet in the air when the cabin pressure could effect static pressure if there is a leak, the error appears.

Flight

Another place to look for leaks, as metioned by phantomphixer, is the ASI case itself and downstream of the ASI. However, if the system passes a static check as leakfree there may be other issues. Has anyone blown out the pitot line? just disconnect at the ASI, and blow low pressure shop air out. Stuff like mud daubbers and wasps and spiders can screw things up royally. I flew one winter with some moisture in the pitot line, and saw everything from 0 to 200mph while doing normal climbout. It is a good exercise at flying by feel, but gets a bit unnerving after awhile.

The Cessna document referenced seems to address only those airplanes where the static port is a second tube co-located with the pitot tube. The standard setup for 170's is a static port located on the left side of the fuselage between the door post and the firewall. As was mentioned in an earlier post the actual static port is riveted to the inside of the fuselage over a simple pin hole in the fuselage skin. On a recent static system check on my airplane, a leak was found and was traced to leakage between the static port and the inside of the fuselage skin. A bead of sealant between the outside edge of the port and the inside of the fuselage skin sealed the system.

There is no doubt that venting the static ports of the airspeed indicator into the cabin will cause an increase in indicated airspeed, as enclosed airplane cabins consistently run at a lower pressure than the actual static air pressure. Plans for the Q1 of the Quickie line of composite homebuilts specified for the static instruments to be vented in the cockpit. That way, you think you're going faster than you really are. Good design on the part of the marketing department.

As was mentioned in an earlier post the actual static port is riveted to the inside of the fuselage over a simple small hole in the fuselage skin. On a recent static system check on my airplane, a leak was found and was traced to leakage between the static port and the inside of the fuselage skin. A bead of sealant between the outside edge of the port and the inside of the fuselage skin sealed the system.

Miles

You fellows have given me a lot of insight. We will check the seal again at the skin of the static port. It may be, that when we checked it, we were not careful enough.

Also, what I find interesting, is what the Phantomphixer said. I read the information from Cessna on the 140. My 1948 170 as we all know is the first year of the airplane (an oversized version of the 140), mine being the 502nd built. When we were refurbishing the airplane we had discovered all the static lines had been removed from the airplane. We thought it strange that the previous owner had done this. We are going to try what Cessna suggests on the Cessna 140 next week. I will let all of you know what the results are at that time.

Thanks so much for your help. Forums like this can be a great resource of information. Engineering is one thing; experience is another. I am grateful for all your experience and willingness to help me resove this problem.

Flight

flight wrote:Engineering is one thing; experience is another.
Flight

Flight,

Can I quote you on that? That one is worthy of an engraved plaque!

Miles

I had promised all of you a posting of the results to my airspeed problem. The Phantomfixer's post gave me the answer. We did as Cessna suggested in the letter to the Dealerships and removed the static line from the ASI only. With the higher q pressure from the cabin, the airspeed indication is now correct at all speeds.

This is not exactly what the FAA likes seeing as a fix to the problem, however, it does work!

I want to thank all of you for your suggestions and help!

Flight